1. You can write everything down.

Question

2. You can write down everything.
(Are both correct?)

2-2. You can write it down. (O)
2-3. You can write down it. (X)

(When the object is a pronoun, we cannot write 'write down it', right? Then, what about #2? In #2, everything is a pronoun. Is #2 correct?)

3. Turn your cellular phone on.
Turn your cellular phone off.
Turn on your cell phone.
Turn off your cell phone.

(Are all correct?)

Answer 1

The first question: both are correct
The second question: huh?
The third question: all are correct

Answer 2

1 and 2 and 2-2 are correct; 2-3 is not correct.

As Nancy said, all in 3 are correct.